KIEV, April 30 (Reuters) - Pro-Russian separatists seized control of state buildings in the town of Horlivka on Wednesday, tightening their grip on swathes of Ukraine's industrial east almost unopposed by police.

The separatists blockaded the region's police headquarters earlier in April, but took over the police division in town and the government administration on Wednesday, a police official told Reuters.

The town of almost 300,000 people sits just north of Donetsk, where mainly Russian-speaking separatists have declared a 'People's Republic' and plan a referendum on secession on May 11.

"They've taken them. The government administration and police," the police official in Donetsk said.

Towns and cities across Ukraine's Donbass coalfield - where giant steel smelters and heavy plants produce around a third of the country's industrial output - have slipped from the control of the pro-Western central government since April 6.

The armed uprising followed the overthrow of Moscow-backed President Viktor Yanukovich and Russia's annexation of Crimea, in a tug-of-war between the West and Russia over the ex-Soviet republic of some 45 million people, a vital transit route for Russian gas to Europe.

The operation in Horlivka follows the fall to separatists on Tuesday of provincial capital Luhansk, further east towards the Russian border. (Reporting by Pavel Polityuk; Writing by Matt Robinson, editing by Elizabeth Piper)